Results
The results from the flow cytometry are given in Table 2, together with the inferred ploidy levels, showing that the Sorbus hybrids (Hedlundia or Aria ) are either triploids or tetraploids.
Size differences in length of the equatorial axis (E) of the measured pollen are shown as boxplot in Figure 2. The mean values vary from 12 – 19.5 µm. The longest E value was measured in A. obtusifolia and shortest equatorial axis is found in H. neglecta . S. aucuparia and A. obtusifolia pollen show the least and most variation, respectively (Figure 2). Comparing the length of E and polyploidy level of the specimens there is a clear correlation between the diploid, triploid, and tetraploid specimens, and length of the equatorial axis (Figure 2).
Measurements of the polar axis (P) are given as boxplots in Figure 3 and show that it is H. neglecta that has the shortest polar axis, andA. edulis , A. obtusifolia and H. hybrida 1 have the longest. The measured means of all specimens vary between 13 and 26 µm. The polar length in H. hybrida 1 grain shows most variation within the measurements. Many specimens have a P between 16 and 20 µm. When comparing the length of P and polyploidy level of the specimens, there is no correlation between the diploid, triploid, and tetraploid specimens, and length of their equatorial or polar axes. But, except forH. hybrida 1, Aria has larger pollen than the other samples tested.
According to Erdtman (1952) the pollen shape can be categorized based on the P/E ratio, and her separates pollen as oblate-spheroidal if the ratio is 0.89–0.99, spheroidal when P/E is 1.00, prolate-spheroidal between 1.01 and 1.14, sub-prolate between 1.15 and 1.33, and prolate if the ratio is higher than 1.34 (Figure 4). The pollen measured in this study all have a mean ratio > 1.00. Following this we see that H. subsimilis , H. meinichii 1+2, H. subpinnata1+2, H. mougeotii , H. neglecta , A. rupicola , andH. subarranensis have a mean ratio between 1.01 and 1.14 and are prolate-spheroidal. H. sognensis , H. lancifolia , H. hybrida 2, and A. obtusifolia have a mean ratio in the sub-prolate group, and H. hybrida 1, S. aucuparia andA. edulis are prolate.
In Figure 4, the ratios are compared to the polyploidy level of the specimens. The two diploid species, A. edulis and S. aucuparia have the largest P/E ratio in this study. The tetraploidH. hybrida 1 has significantly larger pollen compared to the other triploid and tetraploid specimens.
Details of the surface of the pollen grains are not always clear while observing through light microscope (LM), and for that reason, scanning electron microscopy (SEM) was used to observe the surface structures and details of the specimens. Selected images of all our studied specimens are given in Figure 5, showing the polar view, the equatorial view, and details of the surface pattern. The morphological characters of the surface and the corresponding pollen shape for our study specimens are listed in Table 3. Most visible is the long colpi in all our studied specimens, directions and length of striae, number of perforations, and the shape of the pollen grain in polar view.
The PCA ordination for the pollen measures of P and E, and their ratio is given in Figure 6, where 99.91 % of the variation is explained by the two first axes. The diploid S. aucuparia and A. edulishave a clearly higher ratio, as also seen in Figure 4. The tetraploid specimens have a higher value of both P and E, compared to the triploid specimens, but overlapping.
The results from the PCA ordination analysis of all the pollen characters are shown in Figure 7, where 58.68 % of the variation is explained by the two first axes. The diploid species S. aucupariaand A. edulis , have higher values for polar length (P), as also seen in Figure 6, in addition to pollen perforation. The specimens are grouped by genus in this figure and shows that Aria has larger values of the measured characters and are grouped separately fromHedlundia . Hedlundia on the other hand, has large variation between the different specimens, with the extremes H. hybrida 1, H. lancifolia , and H. neglecta .